Plywood is an example of a multi-ply product—that is a product which is made of a plurality of layers of sheet material in which the faces of adjacent sheets are bonded to each other. Plywood is a panel product manufactured by gluing together one or more veneers to both sides of a veneer, solid wood, or reconstituted wood core. In the case of solid-wood-core plywood and reconstituted-wood-core plywood, an additional intermediate step is the production of cores, which are made by lateral gluing of blocks or strips of wood or by gluing oriented wood chips with resin adhesives. Plywood has many advantages over natural wood, an important one being greater dimensional stability. Its uniformity of strength, resistance to splitting, panel form, and decorative value make it adaptable to various uses.
In plywood, both structural and decorative plywood, the grain of alternate layers is typically crossed, in general at right angles and the species, thickness, and grain direction of each layer are matched with those of their opposite number on the other side of the core or central veneer. The total number of layers of veneers is typically odd (three, five, or more) so that the physical properties of the plywood are balanced around its central plane (i.e. the plane parallel to the faces of the plywood and passing though the mid-line of the plywood) i.e. it has approximately the same physical properties (e.g. expansion/contraction with changes of temperature and/or humidity) on both sides of the midline. In other words, the potential for dimensional change when subject to changes of ambient temperature and/or humidity must be equal on the two sides of the panel at equal distances from the centre of the panel. Preferably in moving from the centre of a panel outward toward the surfaces, the panel makeup should be a mirror image on both sides of the panel. Balanced plywood is stable and less likely to cup, warp, shrink or swell. Laminating an odd number of plies [3, 5, 7 . . . ] reduces warping while increasing the number of plies increases the resistance to shearing forces.
Typically the grain direction of the two surface veneers (the front and back faces) is parallel to the longer dimension of rectangular sheets. Three-ply is stronger along the surface grain axis, but as the number of plies increase, the lengthwise/crosswise strengths and stiffness of a plywood sheet will become more equal. Three-ply bends easier along the grain direction of the surface plies because only the middle core ply will have crosswise grain. To ensure the strength and stiffness characteristics of three-ply in the face grain direction it is required that the thickness of each surface veneer is between 25% and 33% of the total sheet thickness, i.e. the two surface sheets comprise 50% to 66% of the total sheet thickness.
The outer plies may be called the ‘faces’ (or the ‘face’ and ‘back’) and the intermediate plies may be called the ‘crossbands’. In order to procure a balanced panel, with five or more plies the total thickness of the odd numbered plies [number 1 being a face ply] should be about the same total thickness as the even numbered plies.
Plywood is often used as a platform for decorative veneers or lamellae used in floor or wall panels. During the manufacture of such floor or wall panels (called “decorative panels” in the following for the sake of brevity, but without implying that the invention is restricted to panels having an attractive decorative veneer or lamella) a decorative veneer or a decorative solid wood sawn lamella, usually of a thickness of 2-4 mm in Europe but as little as 0.2 mm in Asia, of a decorative wood is attached, using by gluing, to one of the faces of a sheet of plywood. This leads to an unbalanced product as the formerly balanced plywood platform has the decorative veneer only attached to one face. When such decorative panels are subjected to changes in relative humidity, e.g. from 10% relative humidity during the winter months to 80-90% relative humidity during the summer, or large changes of temperature, deformations in the decorative surface may occur as the swelling and contraction of the fibres of the decorative veneer cannot be balanced by the underlying plywood platform.